Thanks. I've googled it and it does look possible. Wikipedia says it spreads via burrs on fur and we get a lot of foxes and cats in our garden so it is quite possible that it has come in on one of them.
No there are a few. I've zapped them with weed killer. I'll pull them up at the weekend and see what the roots are like.
I don't know about any of those things. It has only just appeared for the first time. I've had the garden for three years but haven't seen it before.
Does anyone know what this weed is?